With the advent of wireless local area networks (LANs) and mobile devices, such as personal digital assistants, cellular telephones, and laptop computers, users can access the Internet, e-mails, files, and applications from virtually anywhere. Wireless LANs based on the IEEE 802.11 standard have become the predominant option for gaining wireless access to the Internet because of their low cost and unregulated bandwidth.
In recent years, much excitement has developed regarding the prospect of having a mobile device with the capability of the voice over internet protocol (sometimes referred to herein as “Voice over IP” or “VoIP”) and similar applications used over an IEEE 802.11-based wireless LAN. VoIP allows voice and potentially other multimedia data types to be delivered between two or more parties over a network, such as the Internet, rather than traditional telephone lines. Data, whether computer information or audio data, is sent digitally in packed using the internet protocol. There is a great deal of public interest in this notion of having a mobile device that uses a common infrastructure to handle these types of data. For example, users are already attempting to make inexpensive long distance phone calls through the Internet using wireless LAN phones.
VoIP and other applications are problematic, however, due to the latency of the handoff process that is introduced when a mobile device moves from one access point (AP) to another. The handoff process generally involves two logical phases: a discovery phase, which includes handoff initiation and scanning, and a reauthentication phase, which includes authentication and association with a new access point. The amount of time needed to complete this handoff process is a sufficient amount that it causes broken speech, static, and/or dropped calls during VoIP communications.
Several attempts have been made to reduce handoff latency. One solution has been proposed that focuses on reducing the reassociation delay by using a caching mechanism located at the access point. (See, e.g., Mishra et al., “Context caching using neighbor graphs for fast handoffs in a wireless network,” Computer Science Technical Report CS-TR-4477, University of Maryland, February 2004.) This caching mechanism is based on the Inter Access Point Protocol (LAPP) and is used to exchange client context information between neighboring access points. The cache in the access point is filled using the information contained in LAPP Move-Notify messages or reassociation requests sent to the access point by mobile devices. By exchanging the client context information with the old access point, the new access point does not require the client to send its context information in order to reassociate, thereby reducing the reassociation delay.
Another solution has been proposed that focuses on the 802.11 authentication process. (See, e.g., S. Pack et al., “Fast inter-AP handoff using predictive authentication scheme in a public wireless LAN,” in Proc. IEEE Networks 2002 (Joint ICN 2002 and ICWLHN 2002), August 2002.) The authentication process is generally performed after the mobile device has already associated with a new access point. The IEEE 802.11 authentication delay is reduced by using a Frequent Handoff Region (FHR) selection algorithm that takes into account users' mobility patterns, service classes, etc.
However, while these solutions reduce the delay in the reauthentication phase, the discovery phase, and especially the scanning portion of the discovery phase, is the most time consuming part of the handoff process. The scanning portion attributes for over 90% of the total handoff latency, while the reassociation time is generally only a few milliseconds.
Other solutions have been proposed, such as the use of a selective scanning algorithm. (See, e.g., S. Kim et al., “Selective channel scanning for fast handoff in wireless LAN using neighbor graph,” the 2004 International Technical Conference on Circuits/Systems, Computers and Communications (ITC-CSCC2004), Japan, Jul. 6-8, 2004.) However, this approach requires the use of neighbor graphs, multiple changes must be made to the network infrastructure, and IAPP must be used. In addition, this approach defines the scanning delay as the duration taken from the first probe request message to the last probe request message. This definition does not take into account the time needed by the client or mobile device to process the received probe responses. The processing performed by the client represents a significant portion of the scanning delay and increases significantly with the number of probe responses received by the mobile device.
Therefore, there is a need in the art for systems and methods for reducing the handoff latency in wireless networks by reducing the scanning time. Accordingly, it is desirable to provide systems and methods that overcome these and other deficiencies of the prior art.